Trying to export [[Wikipedia:WikiProject_Fungi/fungus_articles_by_size]] (bug 24675), which is subject to a fatal, you get an XML whose tail is:
#[[Scleroderma cepa]] is 203 bytes</text>
</revision>
<!doctype html>
<html><head><title>Internal error</title></head><body>
<div class="errorbox">[7aae7e6a] 2012-08-24 19:09:20: Fatal exception of type MWException</div>
<!-- Set $wgShowExceptionDetails = true; at the bottom of LocalSettings.php to show detailed debugging information. --></body></html>
This, as well as bug 29961 (T31961), might be a blocker for bug 5701 (T7701) and for any piece of software (and human) expecting some error message when export fails.
Version: 1.20.x
Severity: critical